This invention relates generally to cathodes for cathodoluminescent display devices and particularly to a low power cathode having high dimensional precision and low thermal expansion.
Kinescopes for color television receivers and flat panel display devices, such as that described in U.S. Pat. No. 4,031,427, generate a visual display by scanning a phosphor screen with electrons. The electrons are provided by a cathode which is coated with emissive material. The cathode is heated, either directly or indirectly, to cause the emissive material to emit electrons. The electrons are passed through a focusing structure which focuses them into beams. The beams are modulated with the video information to produce the desired display on the phosphor screen as the beams are scanned across the screen. Directly heated cathodes are advantageous because they have low power consumption and short warm up times. However, in such cathodes the heating current passes directly through the cathode. However it is difficult to make a low power cathode that is precisely located with respect to the grids. The reason is that heat is lost thru the conducting parts which carry the heater current. Extending the conductors increases their length and thermal impedance, thereby lowering heat loss. But small location errors of the conductors are multiplied by the long `lever arm` and location accuracy of the cathode is lost. Likewise rigidity of the supporting conductors decreases, thereby decreasing the location accuracy of the cathode. Reducing the cross section of the conductors to increase the thermal impedance likewise reduces rigidity, and lowers the precision with which the cathode can be located with respect to the grids. In addition, the conducting members that carry cathode heater current thermally expand as the temperature rises. The cathode support members, therefore, thermally expand and contract as the cathode heats and cools. The continual thermal expansion and contraction produces a loss in the location accuracy of the cathode, and may cause permanent thermal deformation which adversely affects the operational characteristics of the cathode and which thus also degregates the operational characteristics of the tube. There therefore is a need for a low power cathode having high dimensional precision and low thermal expansion while simultaneously being easy and inexpensive to manufacture. The instant invention fulfills these needs.